The present invention relates to a process for the production of polyester polyols useful in the production of rigid foams.
It is known to prepare rigid polyurethane foams by the reaction of a polyisocyanate with a hydroxyl-terminated polyester, polyether or polyamide. However, the properties of rigid polyurethane foams can differ markedly depending upon the composition of the materials reacted. Further, the composition of the materials reacted can have a significant effect upon the ultimate cost of a particular rigid polyurethane foam. Accordingly, there has been constant change within the polyurethane industry to seek new and improved reactants which are useful in the preparation of rigid polyurethane foams having superior properties or physical characteristics or which reduce the cost of rigid polyurethane foam, or both.
In striving to achieve a rigid polyurethane foam having the property of increased fire resistance, it is known that the industry has sought to utilize cellular polymers in which the major recurring polymer is isocyanurate. See, for example, U.S. Pat. Nos. 3,516,950; 3,625,872; 3,644,232; 3,676,380; 3,725,319; 3,745,133; 3,763,057; 3,793,236; 3,799,896; 3,849,349; 3,876,568; 3,891,579 and 3,909,465. However, the physical properties of the foregoing compositions can vary widely as can the cost of such materials. Thus, the usefulness of such materials can vary depending upon the cost to produce the materials as well as the use to which the materials are to be put. In regard to the cost of the materials, the process by which the materials are produced may be a determinative factor in choosing between somewhat analogous materials. In addition, the utilization of a cheap starting material in a suitable process may also be a determinative factor in choosing between somewhat analogous materials.
A key material in the production of rigid polyurethane foams and several additional end products, is the polyol reactant. The preparation of polyester polyols by the reaction of polycarboxylic acids, anhydrides or esters of polycarboxylic acids with polyhydric alcohols is well known. Generally, the processes of the prior art involve a one-step reaction of the polycarboxylate source with a stoichiometric excess of a polyhydric alcohol. A sufficient excess of alcohol is particularly employed to provide polyesters with low acid numbers.
One of the problems associated with the prior art is in utilizing the large and expensive reactors required to produce the reaction materials in an optimum manner. In the normal procedure for the preparation of polyester polyols, all the reactants are charged to a reactor, and as the reaction proceeds, the water or other volatile product of the reaction is removed from the reactor by distillation. With this procedure, however, only a limited proportion of the contents actually participate in producing the volatile by-product.
Another problem resulting from charging all the reactants at once is that materials that dissolve or digest with difficulty grossly impede the agitation in the reactor and extend the processing and reaction times.
If a high melting product, such as polyethylene terephthalate, for example, is added to the product of the first step, other problems develop. The time of solution at temperatures too low can be excessive and increase processing costs. If higher temperatures are used, excessive loss of glycols through volatilization or decomposition results and causes loss of color.
For examples of prior processes, see, by way of partial example only, U.S. Pat. Nos. 3,162,616; 3,716,523; 3,846,347; 3,865,806; 3,884,850; 3,907,863 and 4,018,815 and Canadian Pat. Nos. 1,127,636 and 1,150,315.
In an attempt to solve at least some of the problems associated with the prior art, U.S. Pat. No. 4,223,068, the disclosure of which is specifically incorporated herein by reference, described a method for making rigid polyurethane foam wherein from 5 to 30% of the weight of an organic polyol is a digestion product of polyalkylene terephthalate. The use of a digestion product prepared by digesting polyalkylene terephthalate residues in organic polyols was described as providing unexpected properties and reduced product cost in U.S. Pat. No. 4,417,001, the disclosure of which is specifically incorporated herein by reference. However, while both of these patents described the improved properties and lower cost attained through the use of digested polyalkylene terephthalate, a need for a very efficient process for the preparation of such compounds was not widely recognized. In U.S. Pat. No. 4,439,550, the disclosure of which is specifically incorporated herein by reference, the use of scrap polyalkylene terephthalate was reviewed. However, the prior art has been concerned with the actual use of a polyaklyene terephthalate in known processes rather than in more efficient processes which can utilize the advantages of a polyalkylene terephthalate. Further, processes to vary the physical characteristics of the materials derived from the same reactants have been, for the most part, either ignored or relegated little attention. Thus, while a batch process could be altered depending upon desired characteristics, such alteration probably would not maximize the use of the chemical production equipment in use.
Accordingly, there exists a need for processes which allow easy alteration of the physical properties of materials useful in the manufacture of rigid polyurethane foams at a minimal cost. In addition, a need also exists for processes for preparing polyester polyols which minimize cost by providing improved utilization of reactor capacity, improved quality of the final product and increased yield.